Photosensitive phosphoproteins in Halobacteria: regulatory coupling of transmembrane proton flux and protein dephosphorylation.

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Abstract
A photoregulated reversible protein phosphorylation system controlled by the halobacterial [Halobacterium halobium] rhodopsins was recently reported. The initial steps in the pathway from the absorption of light to the photoregulated protein phosphorylation and dephosphorylation reactions were identified. Action spectrum, and biochemical and genetic analyses show that the proton pump bacteriorhodopsin mediates light-induced dephosphorylation of 3 photoregulated phosphoproteins. Light absorbed by bacteriorhodopsin is used to establish a proton efflux from the cells. The increase in the inwardly directed protonmotive force (pmf) from this efflux induces dephosphorylation of the 3 phosphoproteins, as demonstrated by the effects of the protonophore CCCP [carbonyl cyanide m-chlorophenyl hydrazone] and of artificially imposed transmembrane pH gradients. Upon darkening the cells, cessation of the protein efflux through bacteriorhodopsin causes a decrease in pmf, which induces rephosphorylation of the proteins. Pmf appears to function as a regulator rather than a driving force in this system. Measurements of pmf-driven ATP synthesis in these conditions indicate the regulation of protein phosphorylation by pmf probably is not a consequence of proton flux through the H+-ATPase, a known energy coupling structure in these cells. The properties of this system may indicate the existence of a pmf detector which regulates kinase or phosphatase activity, i.e., a regulatory coupling device.